PON technology is one of the fiber-to-the home (FTTH) technologies suggested to effectively provide a necessary band for a subscriber terminal. As illustrated in FIG. 4, a conventional PON is usually configured in a point-to-multipoint structure in which a plurality of optical network units (ONUs) 402 commonly share one OLT 401 through a passive device such as an optical distributor 403. Standardization association groups including International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) and Institute of Electrical and Electronics Engineers (IEEE) have standardized asynchronous transfer mode-passive optical network (ATM-PON), Ethernet passive optical network (E-PON) and Gigabit passive optical network (G-PON) as a FTTH network having such a PON structure.
An optical transfer network between the OLT 401 and the corresponding ONUs 402 is called an optical distribution network (ODN), and the ODN is configured in a tree structure.
If the ODN structure is formed in multiple steps, the OLT 401 and the individual ONUs 402 are spaced away from each other, or these two cases co-exist, a signal level is likely attenuated, and thus, maximum optical power is often required. For this reason, when data are transmitted from the OLT 401, it is generally mandated to use an optical signal at maximum power.
However, because of the characteristics of the PON, which can have various ODN structures and variable distances to the ONUs, an optical signal at maximum power is usually unnecessary in most cases. Even though all of the ONUs 402 connected to the OLT 401 turn off during the operation of the PON, the OLT 401 transmits the optical signal, thereby increasing unnecessary power consumption.
As one example of the conventional optical power control methods, Korean patent laid-open number 2005-0051997 disclosed on Jun. 2, 2005 in the name of “Remote Power Control Apparatus and Method for PON System” proposes a method and apparatus for remote controlling the intensity of a burst mode signal that ONUs transmit to an OLT in an E-PON. In detail, a signal intensity detector that detects the intensity of a received signal at a receiving unit of the OLT is provided to help the OLT to control an appropriate power intensity of an optical signal of each of the ONUs, so that the magnitude of the optical signal received at the OLT is uniform.
As another example, Korean patent laid-open number 2005-0005602 disclosed on Jan. 14, 2005 in the name of “Optical Signal Power Level Measuring Apparatus and OLT (Optical Line Termination) Having the Apparatus in Gigabit passive optical network” suggests an apparatus for measuring an optical signal power level in a G-PON and an OLT including the same apparatus. Due to the characteristics of the G-PON, optical power is adjusted when the optical power level of an ONU is out of a predetermined range, and not adjusted when the optical power level is within the pre-determined range. On the basis of this fact, the intensity of an optical signal is not measured precisely but compared with a predetermined critical level. If necessary, an indication signal for adjusting the power level is generated.
As described above, the optical power control in the typical PON proceeds by which the OLT adjusts the optical power at the ONU side to be in an appropriate range. Hence, a range of the optical signal that has to be processed at an OLT receiver decreases to reduce the power consumption and errors. However, optical power control at an OLT transmission side has not yet been suggested.